1. Field of the Invention
The present invention generally pertains to the art of heating pipelines and, more particularly, to heat-tracing systems of the proximity-effect type for heating long-distance pipelines.
2. State of the Art
In long-distance pipelines for transporting viscous fluids such as heavy oil and molten sulfur, it is known that flow may be faciliated by heating the fluids to temperatures above ambient. To provide such heating, it is also known to utilize so-called heat-tracing pipes, which run lengthwise in contact with the pipelines. Further, it is known that the heat-tracing pipes can be heated by electrical means to provide conduction of thermal energy to the main pipeline.
A type of electrical heating conventionally used in heat-tracing systems is known as proximity-effect heating. Such heating is accomplished by installing insulated cable which runs lengthwise through the interior of a ferromagnetic heat-tracing pipe for connection to the pipe at a location remote from a source of alternating current connected to the insulated cable. In practice, alternating current sources are located at convenient intervals, usually ranging from a few hundred to a few thousand feet, along pipeline. In the proximity-effect circuits, alternating current flows from the insulated cable into the ferro-magnetic pipe and then returns to the alternating current source through the wall of the pipe. The current through the ferromagnetic pipe, because of electromagnetic induction and other effects, concentrates on the inner surface of the pipe; such current concentration is properly referred to as proximity effect heating, although the term "skin effect" is often applied. Heat is generated in such proximity-effect circuits primarily by electrical resistance (i.e., I.sup.2 R losses), but also by magnetic hysteresis and by eddy currents. Temperatures may reach about 300.degree. F. or more in the heat-tracing pipes.
In proximity-effect heating systems, the relationships between heat, current and voltage are generally as follows: for a given heat input per unit length of heat-tracing pipe, a certain current is required; to provide the required current in a branch of the proximity-effect circuit, a voltage must be applied directly proportional to the length of the insulated cable defining the branch. Voltages exceeding several kilovolts are required in practice to provide appreciable heating over long distances. However, voltages which can be safely applied to proximity-effect circuits are limited by the rating of insulated cable utilized in the circuits; that is, voltages in excess of the rating of an insulated cable may result in breakdown or disintegration of the insulation surrounding the cable. On the other hand, to increase insulation ratings of cables sufficiently to preclude electrical breakdown at high voltages and high temperature is expensive. The cost of insulated cable for use at voltage levels of about twenty-four kilovolts, for example, has been estimated to comprise about half of the capital cost of a proximity-effect heating system.
To improve existing proximity-effect heating systems, it would be desirable to be able to reduce the voltages required for systems of a given length, or to increase the length of a proximity-effect circuit which can be effectively energized by a given voltage. On approach for lengthening proximity-effect circuits is suggested in U.S. Pat. No. 4,523,177. According to this patent, the secondary winding of a first transformer comprises the power source for a heating circuit which includes the primary winding of a subsidiary transformer. The patentee suggests that additonal lengths of circuit can be provided by adding subsidiary transformers in series.
A method for reducing the cost of electrical insulation in proximity-effect circuits is suggested in U.S. Pat. No. 3,423,966. According to the patentee, the electrical potential between an insulated conductor and a heat-tracing pipe decreases gradually along the length of the heat-tracing pipe, and the grade of insulation utilized in the proximity-effect circuit can be decreased in a stepwise fashion corresponding to decreases in electrical potential in order to economize on insulation costs.